Measurement Of Acoustic Power Of Ultrasound Transducer Based On The Piezoelectric Effect

Ultrasound transducer is the core component of ultrasound equipment, which has been widely used in the field of medical treatment, industry, agriculture, military and so on. Acoustic power is regarded as an important measurement index of acoustic character parameters for describing acoustic energy output by ultrasound transducer, which is the one of most important bases for the evaluation and control of quality of ultrasound equipment. Therefore, the accuracy of acoustic power directly is related to the safety and reliability of ultrasound device. In particular, in the terms of clinical application of focused ultrasound transducer for cancer treatment, the acoustic power setting of transducer is one of key factors which will have a great influence on therapeutic effects and status of disease treatment. In order to make the ultrasound device obtain better control and application, the precision and simplicity of acoustic power measurement technology have become focused direction in the engineering and measurement of ultrasound equipment.There are some problems in the existing measurement methods of acoustic power, such as interference caused by absorbing target and reflecting target for receiving the effect of sound field, high requirement for the axial symmetry and operation complexity of conical reflecting target, and vulnerability to interference of environment. In order to improve the simplicity and quickness, a measurement method of acoustic power based on the piezoelectric effect is proposed in this paper. A piezoelectric ceramic plate is used to directly receive ultrasonic signal, the acoustic signal is converted into electrical signal. And the change of acoustic power is reflected according to the change of output piezoelectric signal. By the means of envelope detection technique, the maximum peak voltage output by piezoelectric ceramic plate is obtained, which is caused by incident acoustic wave before interference of reflected acoustic wave emerging. Then, combined with the numeric conversion between piezoelectric signal and acoustic power, the simple measurement method of acoustic power is realized,which is avoiding the interference caused by reflected acoustic wave.Therefore, in the terms of theory and experiment, the research works in this paper are introduced as follows:1. Starting with the application value of ultrasound transducer and the importance of acoustic power measurement, the advantages and disadvantages of different existing measurement methods of acoustic power are introduced, the key points for enhancing the simplicity and accuracy of measurement technology of acoustic power and the measurement effects expected to reach are clarified.2. The piezoelectric ceramic plate is used to directly receive acoustic wave, due to acoustic wave propagating to the surface of piezoelectric ceramic plate, there will produce reflected acoustic wave. When the reflected acoustic wave propagate to the surface of transducer, the output acoustic power will be affected, then acoustic power will decline with it. As a result, the acoustic force formed on the surface of piezoelectric ceramic plate will fall as well, according to the piezoelectric effect, the piezoelectric signal output at both ends will generate a corresponding changes of amplitude. Therefore, the variation of acoustic power of transmitting transducer is described in electrical signal form.3. Combined with piezoelectric equations and electromechanical analogy, the composing model of piezoelectric signal output by piezoelectric ceramic plate is obtained. The characteristic value of piezoelectric signal for describing the changes of acoustic power is identified. And the theoretical conversion model between output piezoelectric signal and acoustic power of transducer is determined.4. The piezoelectric signal processing is a crucial aspect, the basic principle of envelope detection technique is introduced, the advantages and disadvantages of several methods for extracting the envelope curve of signal are analyzed. Compared to wavelet transform and Hilbert-Huang transform for extracting envelope, the superiorities of local peak point interpolation algorithm are summed up. The voltage amplitude curve characterizing changes of acoustic power is obtained after the envelope of output piezoelectric signal is extracted by local peak point interpolation algorithm, whose variation trend is same to the curve of theoretical analysis.5. Under the different transducer driving voltage, the maximum peak voltages of envelope curve are obtained respectively, whose squared values are linear fitting with acoustic power measured by acoustic power meter which is based on the radiation force, and the proportionality coefficient of theoretical formula is calibrated. Under the different transducer driving voltage, the distribution curves of calibrated acoustic power and that measured by acoustic power meter are almost coincident, which proves the feasibility of the measurement method.